Voltage sensing system with input impedance balancing for electrocardiogram (ECG) sensing applications

ABSTRACT

A voltage sensing system includes input impedance balancing for electrocardiogram (ECG) sensing or other applications, providing immunity to common-mode noise signals while capable of use with two electrodes. Signals are received at first and second electrodes having associated impedances. An impedance circuit includes a feedback controller that adjusts an effective impedance associated with the second electrode based on a difference signal, a common mode signal, a phase-shifted (e.g., quadrature common mode) signal, and an impedance associated with the first electrode. As a result, signals associated with each electrode undergo a similar degree of gain/attenuation and/or phase-shift. This reduces common mode noise and enhances the signal-to-noise characteristics of a desired ECG or other output signal, without requiring the use of more than two electrodes.

CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application is a division of U.S. patent application Ser.No. 10/198,585, filed on Jul. 18, 2003, which is a division of U.S.patent application Ser. No. 09/765,722, filed on Jan. 18, 2001, nowissued as U.S. Pat. No. 6,438,406 which is a continuation of U.S. patentapplication Ser. No. 09/243,265, filed on Feb. 3, 1999, now issued asU.S. Pat. No. 6,208,888, the specifications of which are herebyincorporated by reference.

TECHNICAL FIELD

[0002] This invention relates generally to a voltage sensing system andparticularly, but not by way of limitation, to a voltage sensing systemwith input impedance balancing for electrocardiogram (ECG) sensingapplications.

BACKGROUND

[0003] When functioning properly, the human heart maintains its ownintrinsic rhythm, and is capable of pumping adequate blood throughoutthe body's circulatory system. The body's autonomous nervous systemgenerates intrinsic electrical heart activity signals that are conductedto atrial and ventricular heart chambers on the left and right sides ofthe heart. The electrical heart activity signals trigger resulting heartcontractions that pump blood.

[0004] The intrinsic electrical heart activity signals can be monitoredto provide an electrocardiogram (ECG) signal to a physician, clinician,diagnostician, or researcher to obtain information about heart function.In one such technique, a first external skin patch electrodes isadhesively affixed to the patient's right arm. A second external skinpatch electrode is adhesively affixed to the patient's left arm. Aninstrumentation amplifier is used to detect the electrical heartactivity signals at the first and second electrodes. The instrumentationamplifier outputs an ECG signal based on the difference of the signalsat the first and second electrodes.

[0005] If no further electrodes are used, the ECG signal obtainedbetween the first and second electrodes is typically severely degradedby common-mode (CM) noise signals, such as 60 Hertz or otherenvironmental noise signals that are present at both of the first andsecond electrodes. Common-mode noise problems generally result even if ahigh-quality instrumentation amplifier is used. Skin-electrode interfaceimpedance differences between the first and second electrodes contributeto such common-mode noise problems. Differences in skin-electrodeinterface impedances result from differences in body morphology,adhesion of the electrode, perspiration by the patient, etc. Because ofthe high input-impedance of the instrumentation amplifier, even smalldifferences in the skin-electrode impedance (e.g., 10 kiloohms) canresult in a common-mode noise signal amplitude that exceeds theamplitude of the desired ECG signal.

[0006] One technique of reducing the common-mode noise signal is toattach a third electrode, such as at the patient's right leg, for use ina feedback arrangement. The third electrode is driven by an offsettingcommon-mode signal to cancel a portion of the unwanted common-mode noisesignal. However, this technique is inconvenient for the physician,because it requires attachment of the third electrode to the patient.This increases the complexity of the medical procedure. In a medicalemergency, for example, such increased complexity is highly undesirable.Thus, there is a need for improved ECG measurement techniques providingadequate common-mode noise immunity without relying exclusively onattaching additional electrodes to the patient.

SUMMARY

[0007] The present system provides, among other things, a voltagesensing system with input impedance balancing for electrocardiogram(ECG) sensing or other applications. The present system allows sensingof ECG or other input voltage signals and reduces sensing of unwantedcommon-mode noise signals. The present system is capable of use with twoelectrodes, while still providing good signal-to-noise characteristics.

[0008] According to one aspect of the present system, signals arereceived at first and second electrodes or terminals, each having animpedance associated therewith. An effective impedance associated withthe second electrode is adjusted based on an effective impedanceassociated with the first electrode. In one embodiment, an impedancecircuit adjusts the effective impedance associated with the secondelectrode based on difference and common mode signals obtained fromsignals at the first and second electrodes. As a result, signalsassociated with each electrode undergo a similar degree ofgain/attenuation and/or phase-shift. This reduces common mode noise andenhances the signal-to-noise characteristics of a desired ECG or otheroutput signal, without requiring the use of more than two electrodes.Thus, in an ECG signal acquisition application, the present systemenhances the noise immunity of the ECG signal without increasing thecomplexity of the associated medical procedure. Other aspects of theinvention will be apparent on reading the following detailed descriptionof the invention and viewing the drawings that form a part thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In the drawings, like numerals describe substantially similarcomponents throughout the several views.

[0010]FIG. 1 is a schematic/block diagram illustrating generally oneembodiment of portions of a voltage sensing system and an environment inwhich it is used.

[0011]FIG. 2 is a schematic/block diagram that illustrates generally oneembodiment of portions of a voltage sensing system, such as an ECGdetector, and an environment in which it is used.

[0012]FIG. 3A is a schematic diagram illustrating generally oneembodiment of a first input circuit.

[0013]FIG. 3B is a schematic diagram illustrating generally anotherembodiment of a first input circuit.

[0014]FIG. 4A is a schematic diagram illustrating generally oneembodiment of a second input circuit.

[0015]FIG. 4B is a schematic diagram illustrating generally anotherembodiment of a second input circuit.

[0016]FIG. 5A is a schematic diagram illustrating generally oneembodiment of a configuration of a first amplification circuit and anaverager.

[0017]FIG. 5B is a schematic diagram illustrating generally oneembodiment of a merged first amplification circuit and averager.

[0018]FIG. 6A is a schematic/block diagram illustrating generally oneembodiment of an impedance circuit.

[0019]FIG. 6B is a schematic/block diagram illustrating generally oneembodiment of a feedback controller circuit portion of the impedancecircuit.

[0020]FIG. 7 is a schematic diagram illustrating generally oneembodiment of an impedance control subcircuit.

[0021]FIG. 8A is a signal waveform diagram illustrating generally oneembodiment of operating a feedback controller circuit in which afiltered ECG signal is substantially in phase with a filtered commonmode signal.

[0022]FIG. 8B is a signal waveform diagram illustrating generally oneembodiment of operating a feedback controller circuit in which afiltered ECG signal is substantially 180 degrees out of phase with afiltered common mode signal.

[0023]FIG. 9A is a signal waveform diagram illustrating generally oneembodiment of operating a feedback controller circuit in which afiltered ECG signal is substantially in phase with a filteredphase-shifted common mode signal.

[0024]FIG. 9B is a signal waveform diagram illustrating generally oneembodiment of operating a feedback controller circuit in which afiltered ECG signal is substantially 180 degrees out of phase with afiltered phase-shifted common mode signal.

[0025]FIG. 10 is a computer simulation signal waveform diagram showingan ECG output signal (where electrode impedances are mismatched) beforeand after activation of the impedance circuit.

DETAILED DESCRIPTION

[0026] In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims and their equivalents. In thedrawings, like numerals describe substantially similar componentsthroughout the several views.

[0027] In this document, the term gain is understood to refer to bothgains greater than one and gains that are less than or equal to one(i.e., the term gain includes attenuation). Similarly, the termamplification is understood to include both gains greater than one andgains that are less than or equal to one. Furthermore, amplificationrefers to amplification of differential mode signals and/oramplification of common mode signals. Amplifier is understood toincorporate the above understanding of amplification.

General System Overview

[0028] This document describes, among other things, a voltage sensingsystem with input impedance balancing for electrocardiogram (ECG)sensing or other applications. The present system allows sensing of ECGor other input voltage signals and reduces sensing of unwantedcommon-mode noise signals. The present system does not require the useof more than two electrodes. However, it is understood that more thantwo electrodes can be used in the present system such as, for example,to further improve its signal-to-noise ratio.

[0029]FIG. 1 is a schematic/block diagram illustrating generally, by wayof example, but not by way of limitation, one embodiment of portions ofthe present voltage sensing system and an environment in which it isused. In FIG. 1, a voltage sensing system includes, for example, an ECGdetector 100. The ECG detector 100 is coupled, via leadwires orotherwise, to input terminals, such as first and second electrodes110A-B located at or communicatively coupled to a living organism, suchas human or other patient 105. In one embodiment, first electrode 110Ais disposed at or near a right arm of patient 105 and second electrode110B is disposed at or near a left arm of patient 105. First and secondelectrodes 110A-B are optionally skin patch electrodes that are affixedto the patient's skin, such as using a conductive adhesive or otherwise.Although the embodiment illustrated in FIG. 1 utilizes externalelectrodes 110A-B, it is understood that other embodiments of thepresent voltage sensing system use electrodes that are implanted inpatient 105.

[0030] In one embodiment, ECG detector 100 is optionally included in acardiac rhythm management system. In one such example, the cardiacrhythm management system also includes an implanted cardiac rhythmmanagement device 115, such as a pacer, a defibrillator, or apacer/defibrillator. The implanted device 115 is coupled to heart 120,such as by one or more leadwires or otherwise, for delivering cardiacrhythm management therapy (e.g., electrical pulses or defibrillationcountershocks). In one embodiment, the cardiac rhythm management systemfurther includes art external programmer 125. A communication device,such as telemetry device 130, communicatively couples externalprogrammer 125 to implanted device 115. Programmer 125 includes ECGdetector 100.

[0031]FIG. 2 is a schematic/block diagram that illustrates generally, byway of example, but not by way of limitation, one embodiment of portionsof a voltage sensing system, such as ECG detector 100, and anenvironment in which it is used. In FIG. 2, body voltages including anelectrical heart activity signal are received at first and secondelectrodes 110A-B, which are modeled schematically. First electrode 110Ahas an effective skin-electrode impedance modeled by resistor 200A inparallel with capacitor 205A. Similarly, second electrode 110A has aneffective skin-electrode impedance modeled by resistor 200B in parallelwith capacitor 205B. Electrodes 110A-B are coupled, at respective nodes210A-B, to respective first and second input circuits 215A-B associatedwith ECG detector 100. Input circuits 215A-B provide outputs atrespective nodes 220A-B. Nodes 220A-B are each coupled to both of firstamplification circuit 225 and averager 230.

[0032] An output of first amplification circuit 225, at node 235,provides an ECG signal output and is coupled to impedance circuit 239.An output of averager 230, at node 245, provides a common mode signalthat is coupled to impedance circuit 239. At least one output ofimpedance circuit 239 is coupled to second input circuit 215B forcontrolling its impedance to reduce the common mode noise signal at theECG signal output node 235.

[0033] As discussed above, the effective impedances of first electrode110A and second electrode 110B may be different. This causes the amountof signal attenuation from the input of electrode 110A to node 210A tobe different from the amount of signal attenuation from the input ofelectrode 110B to node 210B. According to prior art techniques, thisresulted in an unwanted common-mode noise signal amplitude, at node 235,that exceeds the desired ECG signal amplitude at node 235. According toone aspect of the present system, however, impedance circuit 239substantially offsets, corrects, or compensates for effects of theimpedance mismatch between electrodes 110A-B. As a result, the effectivesignal attenuation from the input of electrode 110A to node 220A isapproximately equal to the effective signal attenuation from the inputof electrode 110B to node 210B. This, in turn, decreases the common-modenoise at ECG signal output node 235, such that the desired ECG signal ismore readily discernable at node 235.

EXAMPLES OF INPUT CIRCUITS

[0034]FIG. 3A is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of first inputcircuit 215A. The input signal from first electrode 110A is received atnode 210A through series protection resistor 300A. Resistor 300A limitsa current received by subsequent circuits when high energy is received,such as from electrostatic discharges (ESD) or from the delivery of adefibrillation countershock to heart 120. This protects such circuitsagainst possible damage. Similarly, protection diodes 305A and 310Aclamp the voltage at node 315A, such that it does not exceed thepositive power supply voltage, V_(DD), at node 320, by more than a diodevoltage, and such that the voltage at node 315A does not fall below thenegative power supply voltage, V_(SS), at node 325, by more than a diodevoltage.

[0035] In one embodiment, input circuit 215A also includes a phaseshifter 330A. In one example, phase shifter 330A includes a series phaselead network formed by resistor 335A in parallel with capacitor 340A. Anoutput of phase shifter 330A is coupled, at node 345A, to a positiveinput of a buffer such as that of buffer amplifier 350A. An output, atnode 220A, of amplifier 350A is fed back to its negative input. Theoutput at node 220A of amplifier 350A is also fed back to its positiveinput through feedback resistor 355 and input resistor 360A. Anintermediate node 365A, between series-connected feedback resistor 355and input resistor 360A, is coupled to a stable reference voltage, suchas a ground node, through resistor 370. Input capacitor 375A is coupledbetween the positive input, at node 345A, of amplifier 350A, and theground node.

[0036] Amplifier 350A and the network of resistors 355, 360A, and 370form an impedance bootstrap circuit that effectively increases theeffective impedance of input resistor 360A, as seen at node 345A, ascompared what such impedance would be if resistor 360A directly couplednode 345A to the ground node. The impedance bootstrap circuit operatessuch that an increase in voltage at node 345A results in an increase involtage at nodes 220A and 365A. This reduces the voltage across resistor360A which, in turn, reduces the current through resistor 360A. Becausethe resulting current through resistor 360A, in response to a givenchange in voltage at node 345A, is less than it would be if resistor360A directly coupled node 345A to ground, Ohm's Law indicates that theeffective impedance seen at node 345A is increased. Similarly, adecrease in voltage at node 345A results in a decrease in voltage atnodes 220A and 365A which, in turn, also reduces the current throughresistor 360A, thereby increasing the effective resistance of resistor360A as seen at node 345A.

[0037]FIG. 4A is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation one embodiment of second inputcircuit 215B. As illustrated in FIG. 4A, second input circuit 215B issimilar to first input circuit 215A. Operation of correspondinglynumbered elements (but with a different suffix letter “B”) is asdescribed with respect to FIG. 3A. In FIG. 4A, however, input resistor360B couples a signal received at node 270A, from impedance circuit 239,to the positive input, at node 345B, of a buffer, such as bufferamplifier 350B. Similarly, input capacitor 375B couples a signalreceived at node 270B, from impedance circuit 239, to the positiveinput, at node 345B, of buffer amplifier 350B.

[0038]FIG. 4A illustrates resistor 360B and capacitor 375B as being partof second input circuit 215B, for convenience of illustratingsimilarities and differences between first and second input circuits215A-B. It is understood, however, that resistor 360B and capacitor 375Bare alternatively regarded as being part of impedance circuit 239 ratherthan as being part of second input circuit 215B, and could alternativelybe illustrated therewith.

[0039] In operation, the voltages at nodes 270A-B are adjusted byimpedance circuit 239 (analogous to operation of the impedance bootstrapcircuit described above with respect to FIG. 3A) to vary the effectiveimpedance of resistor 360B and capacitor 375B such that again/attenuation between first electrode 110A and node 345A isapproximately or substantially equal to a gain/attenuation betweensecond electrode 110B and corresponding node 345B. In one embodiment,this results in an attenuation between first electrode 110A and node220A that is approximately or substantially matched to an attenuationbetween second electrode 10B and corresponding node 220B.

[0040] By increasing the voltage at node 270A, relative to the voltageat node 345B, the effective resistance of input resistor 360B isincreased. By decreasing the voltage at node 270A, relative to thevoltage at node 345B, the effective resistance of input resistor 360B isdecreased. According to one aspect of the present system, the voltage atinput node 270A is controlled by impedance circuit 239 such that theeffective resistance of input resistor 360B matches a resistivecomponent of the effective impedance seen at node 345A of first inputcircuit 215A (when the resistor 200A of first electrode 110A isapproximately equal to the resistor 200B of second electrode 110B andthe capacitor 205A of first electrode 110A is approximately equal to thecapacitor 205B of second electrode 110B).

[0041] By decreasing the voltage at node 270B, relative to the voltageat node 345B, the effective capacitance of input capacitor 375B isincreased. By increasing the voltage at node 270B, relative to thevoltage at node 345B, the effective capacitance of input capacitor 375Bis decreased. According to one aspect of the present system, the voltageat input node 270B is controlled by impedance circuit 239 such that theeffective capacitance of input capacitor 375B matches the reactive(e.g., capacitive) component of the effective impedance seen at node345A of first input circuit 215A (when the resistor 200A of firstelectrode 110A is approximately equal to the resistor 200B of secondelectrode 110B and the capacitor 205A of first electrode 110A isapproximately equal to the capacitor 205B of second electrode 110B).

[0042] The system is described above as including phase-lead networks330A and 330B to accommodate a full range of phase lags introduced byimpedance circuit 239, resistor 360B, and capacitor 375B. Alternatively,phase lead networks 330A and 330B are omitted, and a negative impedancecircuit is used in place of at least one of resistor 360B and capacitor375B, as illustrated in FIGS. 3B and 4B by way of example, but not byway of limitation. In FIG. 4B, for example, an additional capacitor 375Cis included, and capacitor 375B is implemented as a negative capacitancecircuit. In this embodiment, capacitors 375A and 375C each have anapproximately equal nominal capacitance value (“C”), and negativecapacitor circuit 375B has a nominal capacitance value of −2C.Alternatively, capacitor 375A has a nominal capacitance value C,capacitor 375B has a nominal capacitance value 2C, and capacitor 375C isimplemented as a negative capacitance network having a capacitance valueof approximately −C.

EXAMPLES OF DIFFERENTIAL AMPLIFIER, AVERAGER AND PHASE-SHIFTER

[0043]FIG. 5A is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of a configurationof first amplification circuit 225 and averager 230, such as illustratedin FIG. 2. In one embodiment, as illustrated in FIG. 5A, firstamplification circuit 225 includes a differential input, single-endedoutput amplifier, such as an off-the-shelf or other instrumentationamplifier. First amplification circuit 225 receives input signals atnodes 220A-B from first and second input circuits 215A-B, respectively,and outputs an ECG signal at node 235.

[0044] In this embodiment, averager 230 includes a differential input,single-ended output operational amplifier 500. Amplifier 500 includes apositive input that is coupled to a ground node and an output, at node245, that provides a common mode voltage of the signals at nodes 220Aand 220B. The common mode signal at the node 245 is fed back to theinverting input, at node 505, of amplifier 500, such as through feedbackresistor 510. The inverting input of amplifier 500, at node 505, iscoupled via first input resistor 512 to receive a signal, at node 220A,from first input circuit 215A. The inverting input of amplifier 500, atnode 505, is also coupled via second input resistor 515 to receive asignal, at node 220B, from second input circuit 215B. In an alternateembodiment, averager 230 includes a passive network (i.e., without usingoperational amplifier 500) for averaging the signals at nodes 220A-B.

[0045] In this embodiment, first amplifier 225 is configured as aninstrumentation amplifier, which includes first operational amplifier520, second operational amplifier 522 and third operational amplifier524, each having differential inputs and a single-ended output. Anoninverting input of first operational amplifier 520 is coupled tofirst input circuit 215A at node 220A. The output, at node 526, of firstoperational amplifier 520 is fed to the inverting input of thirdoperational amplifier 524 through resistor 528, and is also fed backthrough resistor 530 to the inverting input, at node 529, of firstoperational amplifier 520. A noninverting input of second operationalamplifier 522 is coupled to second input circuit 215B at node 220B. Theoutput, at node 532, of second operational amplifier 522 is fed to thenoninverting input of third operational amplifier 524 through resistor534, and is also fed back to the inverting input, at node 535, of secondoperational amplifier 522 through resistor 536. The inverting input node529 of first operational amplifier 520 is coupled to the inverting inputnode 535 of second operational amplifier 522 through series-coupledresistors 538 and 540. The output of third operational amplifier 524provides the ECG signal at node 235, and is coupled back to theinverting input of third operational amplifier through resistor 542. Thenoninverting input of third operational amplifier 524 is coupled to aground node through resistor 544.

[0046]FIG. 5B is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of a configurationof a merged first amplification circuit 225 and averager 230. In thisembodiment, a single instrumentation amplifier 225 is used, and thecommon mode-signal at node 245 is provided by the common mode output ofthe instrumentation amplifier taken between resistors 538 and 540.

EXAMPLE IMPEDANCE CIRCUIT

[0047]FIG. 6A is a schematic/block diagram illustrating generally, byway of example, but not by way of limitation, one embodiment ofimpedance circuit 239. Impedance circuit 239 receives the ECG signal, atnode 235, the common mode signal, at node 245, and the output, at node220B, of second input circuit 215B. The ECG signal at node 235 isamplified at buffer 602, which provides an output at node 604 that isthen filtered by filter 606, which, in one embodiment, is a bandpassfilter that attenuates frequencies outside the range of approximately6-600 Hz (e.g., single pole rolloff frequencies). This, in turn,provides a filtered ECG signal output at node 608 to feedback controller610. In one alternate embodiment, buffer 602 and filter 606 arecombined. In another alternate embodiment, filter 606 is a highpassfilter.

[0048] The common mode signal at node 245 is amplified at buffer 612,which provides an output at node 614 that is then filtered by filter616, which, in one embodiment, is a bandpass filter that attenuatesfrequencies outside the range of approximately 6-600 Hz. This, in turn,provides a filtered common mode signal output at node 618 to feedbackcontroller 610. In one alternate embodiment, buffer 612 and filter 616are combined. In another alternate embodiment, filter 616 is a highpassfilter.

[0049] The filtered common mode signal output at node 618 is alsoreceived by phase-shifter 620, which provides a filtered phase-shiftedcommon mode signal output at node 622 to feedback controller 610. In oneembodiment, phase-shifter 620 includes an integrator circuit thatincludes differential input, single-ended output operational amplifier624. Amplifier 624 has a positive input, which is coupled to ground, andan output at node 622 that is fed back to its inverting input, at node626, through a feedback capacitor 628. The inverting input of amplifier624 is also coupled, via input resistor 630, to receive the filteredcommon mode output signal, at node 618, from the output of filter 616.Phase-shifter 620 provides a filtered phase-shifted common mode outputsignal, at node 622 (which, in one embodiment, is approximately 90degrees out of phase with the common mode signal at node 245 and is alsoreferred to as a filtered quadrature common mode signal). In analternative embodiment, phase-shifter 620 is configured as adifferentiator, rather than as an integrator (i.e., resistor 630 isconfigured in the feedback path around amplifier 624 and capacitor 628is interposed between nodes 618 and 626).

[0050] Based on the filtered ECG signal at node 608, the filtered commonmode signal at node 618, and the filtered quadrature common mode signalat node 622, feedback controller 610 provides a resistive matchingcontrol signal, at node 632, and a capacitive matching control signal,at node 634, to impedance control subcircuit 636. Impedance controlsubcircuit 636 also receives the output signal, at node, 220B, fromsecond input circuit 215B. Based on these input signals, impedancecontrol subcircuit 636 provides control voltages, at node/bus 270 tosecond input circuit 215B for controlling its impedance to reduce thecommon mode noise signal at the ECG signal output node 235.

EXAMPLE FEEDBACK CONTROLLER CIRCUIT

[0051]FIG. 6B is a schematic/block diagram illustrating generally, byway of example, but not by way of limitation, one embodiment of feedbackcontroller circuit 610. Feedback controller circuit 610 receives thefiltered ECG signal, at node 608, the filtered common mode signal, atnode 618, and the filtered phase-shifted common mode signal at node 622.

[0052] In one embodiment, the filtered common mode signal, at node 618,is phase-detected with respect to the filtered ECG output signal, atnode 608, as described below. The filtered ECG signal at node 608 ismixed or multiplied with the filtered common mode signal, at node 618,by a mixer or multiplier (referred to interchangeably herein) such asanalog multiplier 640, which provides a resulting signal, referred to asan in-phase signal, at node 645. The in-phase signal at node 645 isreceived by low pass filter 650. In one embodiment, low pass filter 650attenuates frequency components above a cutoff frequency ofapproximately 40 Hertz, and provides a resulting low pass filteredin-phase signal, at node 655, to integrator 660. Integrator 660integrates the low pass filtered in-phase signal, providing a resultingresistive-matching control signal, at node 632, to impedance controlsubcircuit 636.

[0053] The filtered phase-shifted common mode signal, at node 622, isphase-detected with respect to the filtered ECG output signal, at node608, as described below. The filtered ECG signal at node 608 is mixed ormultiplied with the filtered phase-shifted common mode signal, at node622, by a mixer or multiplier, such as analog multiplier 665, whichprovides a resulting signal, referred to as a quadrature phase signal,at node 670. The quadrature phase signal at node 670 is received by lowpass filter 675. In one embodiment, low pass filter 675 attenuatesfrequency components above a cutoff frequency of approximately 40 Hertz,and provides the resulting low pass filtered quadrature phase signal, atnode 680, to an integrator, such as inverting integrator 685. Invertingintegrator 685 integrates and inverts the low pass filtered quadraturephase signal, providing a resulting capacitive-matching control signal,at node 634, to impedance control subcircuit 636.

EXAMPLE IMPEDANCE CONTROL SUBCIRCUIT

[0054]FIG. 7 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of portions ofimpedance control subcircuit 636. In this embodiment, impedance controlsubcircuit 636 includes one or more variable gain or similar circuits,such as analog multiplier circuits, or first voltage controlledamplifier (VCA) 700A and second VCA 700B. A negative input of each ofVCAs 700A-B is grounded. A positive input of each of VCAs 700A-B iscoupled to node 220B to receive the output signal from second inputcircuit 215B.

[0055] In one embodiment, the gain of first VCA 700A is adjusted by theresistive-matching control signal received at node 632 from feedbackcontroller circuit 610. The gain of second VCA 700B is adjusted by thecapacitive-matching control signal received at node 634 from feedbackcontroller circuit 610. The gain of respective VCAs 700A-B is increasedfor more positive signals at respective nodes 632 and 634, and decreasedfor more negative signals at respective nodes 632 and 634. First VCA700A provides an output voltage, at node 270A, to resistor 360B insecond input circuit 215B. Second VCA 700B provides an output voltage,at node 270B, to capacitor 375B in second input circuit 215B.

[0056]FIG. 4 illustrates resistor 360B and capacitor 375B as being partof second input circuit 215B, for convenience of illustratingsimilarities and differences between first and second input circuits215A-B. It is understood, however, that resistor 360B and capacitor 375Bare alternatively regarded as being part of impedance control subcircuit636 rather than as being part of second input circuit 215B (or otherportion of impedance circuit 239) and could alternatively be illustratedtherewith.

[0057] In one embodiment, first and second VCAs 700A-B provideindependent impedance bootstraps, as discussed above with respect toamplifier 350A in first input circuit 215A. However, the gain of firstand second VCAs 700A-B is adjusted by feedback controller circuit 610 tocontrol the respective node voltages 270A-B to substantially offset orapproximately correct the impedance mismatch between electrodes 11A-B.As a result, the effective signal attenuation from the input ofelectrode 110A to node 220A is approximately equal to the effectivesignal attenuation from the input of electrode 110B to node 210B. This,in turn, decreases the common-mode noise at ECG signal output node 235,such that the ECG signal is more readily discernable at node 235.

EXAMPLE OPERATION OF IMPEDANCE CIRCUIT

[0058]FIG. 8A is a signal waveform diagram illustrating generally, byway of example, but not by way of limitation, one embodiment ofoperating impedance circuit 239. In FIG. 8A, V₆₀₈ represents anillustrative example of a filtered ECG signal at node 608 and V₆₁₈represents an illustrative example of a filtered common mode signal atnode 618. In the example illustrated in FIG. 8A, V₆₀₈ and V₆₁₈ are inphase with each other. The signals V₆₀₈ and V₆₁₈ are multiplied witheach other at multiplier 640, providing V₆₄₅, a resulting in-phasesignal at node 645. For the illustrated signals V₆₀₈ and V₆₁₈, which arein phase with each other, the resulting in-phase signal at node 645 isfrequency-doubled and positive-valued. The in-phase signal at node 645is filtered by low pass filter 650, which attenuates high-frequencycomponents, resulting in a positive-valued signal V₆₅₅ at node 655. Thelow pass filtered in-phase signal at node 655 is integrated byintegrator 660, resulting in an upward ramping resistive-matchingcontrol signal, V₆₃₂ at node 632. An increase in the resistive-matchingcontrol signal at node 632 increases the gain of first VCA 700A, whichincreases the effective resistance of resistor 360B.

[0059]FIG. 8B is a signal waveform diagram, similar to FIG. 8A, butproviding an illustrative example of signals V₆₀₈ and V₆₁₈ being out ofphase with each other. After multiplication, the resulting in-phasesignal V₆₄₅ at node 645 is frequency-doubled and negative-valued. As aresult, the low pass filtered in-phase signal V₆₅₅ at node 655 is alsonegative-valued. Integration yields a downward rampingresistive-matching control signal V₆₃₂ at node 632, which decreases thegain of first VCA 700A, and decreases the effective resistance ofresistor 360B.

[0060]FIG. 9A is a signal waveform diagram illustrating generally, byway of example, but not by way of limitation, another aspect of oneembodiment of operating impedance circuit 239. In FIG. 9A, V₆₀₈represents an illustrative example of a filtered ECG signal at node 608and V₆₁₈ represents an illustrative example of a filtered phase-shiftedcommon mode signal at node 618. In the example illustrated in FIG. 9A,V₆₀₈ and V₆₁₈ are in phase with each other. The signals V₆₀₈ and V₆₁₈are multiplied with each other at multiplier 665, providing V₆₇₀, aresulting quadrature-phase signal at node 670. For the illustratedsignals V₆₀₈ and V₆₁₈, which are in phase with each other, the resultingquadrature-phase signal at node 670 is frequency-doubled andpositive-valued. The quadrature-phase signal at node 670 is filtered bylow pass filter 675, which attenuates high-frequency components,resulting in a positive-valued signal V₆₈₀ at node 680. The low passfiltered quadrature-phase signal at node 680 is integrated and invertedby inverting integrator 685, resulting in a downward ramping capacitivematching control signal at node 634. A decrease in the capacitivematching control signal at node 634 decreases the gain of second VCA700B, which increases the effective capacitance of capacitor 375B.

[0061]FIG. 9B is a signal waveform diagram, similar to FIG. 9A, butproviding an illustrative example of signals V₆₀₈ and V₆₁₈ being out ofphase with each other. After multiplication, the resultingquadrature-phase signal at node 670 is frequency-doubled andnegative-valued. As a result, the low pass filtered quadrature-phasesignal at node 680 is also negative-valued. Integration and signalinversion by inverting integrator 685 yields an upward rampingcapacitive matching control signal at node 634, which increases the gainof second VCA 700B, and decreases the effective capacitance of capacitor375B.

[0062] FIGS. 8A-B provide illustrative examples of the phaserelationship between the filtered ECG signal at node 608 and thefiltered common mode signal at node 618. According to one aspect ofoperation, impedance circuit 239 provides a negative feedbackconfiguration that tends to minimize the magnitude of the low passfiltered in-phase signal at node 655. This effectively matches theeffective resistance of resistor 360B in second input circuit 215B tothe effective resistance seen at node 345A in first input circuit 215A(when the resistor 200A of first electrode 110A is approximately equalto the resistor 200B of second electrode 110B and the capacitor 205A offirst electrode 10A is approximately equal to the capacitor 205B ofsecond electrode 110B).

[0063] Similarly, FIGS. 9A-B provide illustrative examples of the phaserelationship between the filtered ECG signal at node 608 and thefiltered phase-shifted common mode signal at node 618. Impedance circuit239 provides a negative feedback configuration that tends to minimizethe magnitude of the low pass filtered quadrature phase signal at node680. This effectively matches the effective capacitance of capacitor375B in second input circuit 215B to the effective capacitance seen atnode 345A in first input circuit 215A (when the resistor 200A of firstelectrode 110A is approximately equal to the resistor 200B of secondelectrode 110B and the capacitor 205A of first electrode 110A isapproximately equal to the capacitor 205B of second electrode 110B).

[0064] Even when the resistor 200A of first electrode 110A is notapproximately equal to the resistor 200B of second electrode 110B andthe capacitor 205A of first electrode 110A is not approximately equal tothe capacitor 205B of second electrode 110B, the gain/attenuation fromthe input of electrode 110A to node 345A in first input circuit 215A iskept substantially identical to the gain/attenuation from the input ofelectrode 10B to node 345B in second input circuit 215B. As a result,the gain/attenuation from the input of electrode 110A to node 220A isapproximately equal to the gain/attenuation from the input of electrode10B to node 220B. This, in turn, keeps the common mode noise signal atnode 245 at a reasonably small value, improving the signal-to-noisecharacteristics of the ECG signal at node 235.

EXAMPLE TEST RESULTS

[0065] Operation of one embodiment of a voltage sensing circuit wassimulated using a SPICE computer simulation. The component values thatwere used are listed below (by way of example, but not by way oflimitation).

[0066] First electrode 110A: R_(200A)=26 KΩ, C_(205A)=25 nF. First inputcircuit 215A: R_(300A)=10 KΩ, R_(335A)=10 KΩ, C_(340A)=240 nF,R_(360A)=10 MΩ, C_(375A)=120 pF, R₃₇₀=2.6KΩ, R₃₅₅₌₁ KΩ. Second electrode110B: R_(200B)=20KΩ, C_(205B)=10 nF. Second input circuit 215B:R_(300B)=10 KΩ, R_(335B)=10 KΩ, C_(340B)=240 nF, R_(360B)=12 MΩ,C_(375B)=300 pF. Averager 230: R₅₁₀=100 KΩ, R₅₁₂=50 KΩ, R₅₁₅=50 KΩ.Phase Shifter 620: C₅₃₅=6 nF, R₅₃₀=100 KΩ (configured as adifferentiator). Filters 606 and 616 were configured as high passfilters and each included an RC network where R=10 MΩ and C=10 nF. Lowpass filters 650 and 675 each included an RC network where R=400 KΩ andC=10 nF. Integrators 660 and 685 each included an RC integration timeconstant where R=800 KΩ and C=100 nF.

[0067]FIG. 10 is a computer simulation signal waveform diagram, usingabove-described component values having mismatched electrode impedances,and showing the ECG output signal at node 235. Before time t=1 second,the feedback controller circuit was turned off, and the ECG outputsignal is swamped by common mode-noise. At time t=1 second, theimpedance circuit 239 was activated. As illustrated in FIG. 10, thisinitiated the gain/attenuation matching described above. As a result,the common mode noise signal was substantially reduced, as illustratedin FIG. 10 for times greater than 2 seconds, such that the underlyingECG signal waveform was readily discernable as having goodsignal-to-noise characteristics. The circuit was also resimulated withthe impedance mismatch being incorporated into the opposite electrodes,and obtained similar results.

Conclusion

[0068] The above-described system provides, among other things, avoltage sensing system with input impedance balancing forelectrocardiogram (ECG) sensing or other applications. The presentsystem allows sensing of ECG or other input voltage signals and reducessensing of unwanted common-mode noise signals. The present system doesnot require the use of more than two electrodes. Instead, a common modesignal is generated from the two electrodes, and a feedback networkoperates to minimize the common mode signal. It is understood, however,that more than two electrodes can be used in the present system such as,for example, by including a third electrode that provides feedbackcancellation of the common mode voltage to further improve itssignal-to-noise ratio of the system. It is also understood that signalinversions (such as from inverting integrator 685, for example) can bemoved elsewhere in the signal flow.

[0069] It is to be understood that the above description is intended tobe illustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. An apparatus for sensing signals relative to avoltage at a ground node, the apparatus comprising: first, second,third, fourth, control, and ground nodes; a first series impedancebetween the first and third nodes; a first shunt impedance between thethird and ground nodes; a second series impedance between the second andfourth nodes; a second shunt impedance between the fourth and controlnodes; and a control circuit coupled to the control node for providing acontrol signal such that a first gain between the first and third nodesapproximately matches a second gain between the second and fourth nodes.2. The apparatus of claim 1, further comprising: a differentialamplifier having inputs coupled to the third and fourth nodes and anoutput representative of a differential signal; and an averager havinginputs coupled to the third and fourth nodes and an outputrepresentative of a common mode signal.
 3. The apparatus of claim 2,further comprising: a first electrode coupled to the first node; asecond electrode coupled to the second node; and wherein thedifferential amplifier is adapted to amplify an electrocardiographic(ECG) signal.
 4. The apparatus of claim 2, wherein the control circuitcomprises an impedance circuit having a first input coupled to theoutput of the differential amplifier, a second input coupled to theoutput of the averager, and an output representative of the controlsignal.
 5. The apparatus of claim 4, wherein the impedance circuitcomprises; a first bandpass filter, coupled to the output of thedifferential amplifier, to filter the differential signal; a secondbandpass filter, coupled to the output of the averager, to filter thecommon mode signal; a phase-shifter, coupled to the second bandpassfilter, to phase-shift the filtered common mode signal; a feedbackcontroller coupled to the first bandpass filter, the second bandpassfilter, and the phase-shifter, to derive a resistive matching signal anda capacitive matching signal from the filtered differential signal, thefiltered common mode signal, and the phase-shifted filtered common-modesignal; and an impedance control subcircuit, coupled to the feedbackcontroller, to derive the control signal from the resistive matchingsignal and the capacitive matching signal.
 6. The apparatus of claim 5,wherein the control signal is a voltage signal.
 7. An apparatus forsensing signals relative to a voltage at a ground node, the apparatuscomprising: first, second, third, fourth, control, and ground nodes; afirst series impedance between the first and third nodes; a first shuntimpedance between the third and ground nodes; a second series impedancebetween the second and fourth nodes; a second shunt impedance betweenthe fourth and control nodes; and a control circuit coupled to thecontrol node for providing a control signal such that a first phasebetween the first and third nodes approximately matches a second phasebetween the second and fourth nodes.
 8. The apparatus of claim 7,further comprising: a differential amplifier having inputs coupled tothe third and fourth nodes and an output representative of adifferential signal; and an averager having inputs coupled to the thirdand fourth nodes and an output representative of a common mode signal.9. The apparatus of claim 8, further comprising: a first electrodecoupled to the first node; a second electrode coupled to the secondnode; and wherein the differential amplifier is adapted to amplify anelectrocardiographic (ECG) signal.
 10. The apparatus of claim 8, whereinthe control circuit comprises an impedance circuit having a first inputcoupled to the output of the differential amplifier, a second inputcoupled to the output of the averager, and an output representative ofthe control signal.
 11. The apparatus of claim 10, wherein the impedancecircuit comprises; a first bandpass filter, coupled to the output of thedifferential amplifier, to filter the differential signal; a secondbandpass filter, coupled to the output of the averager, to filter thecommon mode signal; a phase-shifter, coupled to the second bandpassfilter, to phase-shift the filtered common mode signal; a feedbackcontroller coupled to the first bandpass filter, the second bandpassfilter, and the phase-shifter, to derive a resistive matching signal anda capacitive matching signal from the filtered differential signal, thefiltered common mode signal, and the phase-shifted filtered common-modesignal; and an impedance control subcircuit, coupled to the feedbackcontroller, to derive the control signal from the resistive matchingsignal and the capacitive matching signal.
 12. The apparatus of claim11, wherein the control signal is a voltage signal.
 13. An apparatus forsensing signals relative to a voltage at a ground node, the apparatuscomprising: first, second, third, fourth, control, and ground nodes; afirst series impedance between the first and third nodes; a first shuntimpedance between the third and ground nodes; a second series impedancebetween the second and fourth nodes; a second shunt impedance betweenthe fourth and control nodes; and a means, coupled to the control node,for providing a control signal such that a first gain between the firstand third nodes approximately matches a second gain between the secondand fourth nodes.
 14. The apparatus of claim 13, further comprising: adifferential amplifier having inputs coupled to the third and fourthnodes and an output representative of a differential signal; and anaverager having inputs coupled to the third and fourth nodes and anoutput representative of a common mode signal.
 15. The apparatus ofclaim 14, further comprising: a first electrode coupled to the firstnode; a second electrode coupled to the second node; and wherein thedifferential amplifier is adapted to amplify an electrocardiographic(ECG) signal.
 16. The apparatus of claim 14, wherein the control circuitcomprises an impedance circuit having a first input coupled to theoutput of the differential amplifier, a second input coupled to theoutput of the averager, and an output representative of the controlsignal.
 17. The apparatus of claim 16, wherein the impedance circuitcomprises; a first bandpass filter, coupled to the output of thedifferential amplifier, to filter the differential signal; a secondbandpass filter, coupled to the output of the averager, to filter thecommon mode signal; a phase-shifter, coupled to the second bandpassfilter, to phase-shift the filtered common mode signal; a feedbackcontroller coupled to the first bandpass filter, the second bandpassfilter, and the phase-shifter, to derive a resistive matching signal anda capacitive matching signal from the filtered differential signal, thefiltered common mode signal, and the phase-shifted filtered common-modesignal; and an impedance control subcircuit, coupled to the feedbackcontroller, to derive the control signal from the resistive matchingsignal and the capacitive matching signal.
 18. The apparatus of claim17, wherein the control signal is a voltage signal.
 19. An apparatus forsensing signals relative to a voltage at a ground node, the apparatuscomprising: first, second, third, fourth, control, and ground nodes; afirst series impedance between the first and third nodes; a first shuntimpedance between the third and ground nodes; a second series impedancebetween the second and fourth nodes; a second shunt impedance betweenthe fourth and control nodes; and a means, coupled to the control node,for providing a control signal such that a first phase between the firstand third nodes approximately matches a second phase between the secondand fourth nodes.
 20. The apparatus of claim 19, further comprising: adifferential amplifier having inputs coupled to the third and fourthnodes and an output representative of a differential signal; and anaverager having inputs coupled to the third and fourth nodes and anoutput representative of a common mode signal.
 21. The apparatus ofclaim 20, further comprising: a first electrode coupled to the firstnode; a second electrode coupled to the second node; and wherein thedifferential amplifier is adapted to amplify an electrocardiographic(ECG) signal.
 22. The apparatus of claim 20, wherein the control circuitcomprises an impedance circuit having a first input coupled to theoutput of the differential amplifier, a second input coupled to theoutput of the averager, and an output representative of the controlsignal.
 23. The apparatus of claim 22, wherein the impedance circuitcomprises; a first bandpass filter, coupled to the output of thedifferential amplifier, to filter the differential signal; a secondbandpass filter, coupled to the output of the averager, to filter thecommon mode signal; a phase-shifter, coupled to the second bandpassfilter, to phase-shift the filtered common mode signal; a feedbackcontroller coupled to the first bandpass filter, the second bandpassfilter, and the phase-shifter, to derive a resistive matching signal anda capacitive matching signal from the filtered differential signal, thefiltered common mode signal, and the phase-shifted filtered common-modesignal; and an impedance control subcircuit, coupled to the feedbackcontroller, to derive the control signal from the resistive matchingsignal and the capacitive matching signal.
 24. The apparatus of claim23, wherein the control signal is a voltage signal.
 25. An apparatus forsensing signals relative to a voltage at a ground node, the apparatuscomprising: first, second, third, fourth, control, and ground nodes; afirst series impedance between the first and third nodes; a first shuntimpedance between the third and ground nodes; a second series impedancebetween the second and fourth nodes; a second shunt impedance betweenthe fourth and control nodes; and a control circuit coupled to thecontrol node for providing a control signal such that a first gain andphase between the first and third nodes approximately matches a secondgain and phase between the second and fourth nodes.
 26. The apparatus ofclaim 25, further comprising: a differential amplifier having inputscoupled to the third and fourth nodes and an output representative of adifferential signal; and an averager having inputs coupled to the thirdand fourth nodes and an output representative of a common mode signal.27. The apparatus of claim 26, further comprising: a first electrodecoupled to the first node; a second electrode coupled to the secondnode; and wherein the differential amplifier is adapted to amplify anelectrocardiographic (ECG) signal.
 28. The apparatus of claim 26,wherein the control circuit comprises an impedance circuit having afirst input coupled to the output of the differential amplifier, asecond input coupled to the output of the averager, and an outputrepresentative of the control signal.
 29. The apparatus of claim 28,wherein the impedance circuit comprises; a first bandpass filter,coupled to the output of the differential amplifier, to filter thedifferential signal; a second bandpass filter, coupled to the output ofthe averager, to filter the common mode signal; a phase-shifter, coupledto the second bandpass filter, to phase-shift the filtered common modesignal; a feedback controller coupled to the first bandpass filter, thesecond bandpass filter, and the phase-shifter, to derive a resistivematching signal and a capacitive matching signal from the filtereddifferential signal, the filtered common mode signal, and thephase-shifted filtered common-mode signal; and an impedance controlsubcircuit, coupled to the feedback controller, to derive the controlsignal from the resistive matching signal and the capacitive matchingsignal.
 30. The apparatus of claim 29, wherein the control signal is avoltage signal.